Usefulness of Influenza Vaccination During Pregnancy

Summary of Methods & Results

The prospective cohort study by Fisher et al. evaluated immunogenicity against the 2009 novel influenza A (H1N1) virus among 40 pregnant women: 16 with confirmed 2009 H1N1 influenza virus infection during pregnancy (infected), 14 women vaccinated against the 2009 H1N1 influenza virus during pregnancy (vaccinated) and ten control women without infection or vaccination (control).^[6] The infected group women were diagnosed by either the rapid antigen influenza A test or a PCR test on respiratory specimens during the 2009–2010 influenza season. The vaccinated group women did not develop influenza-like illness during pregnancy. Maternal and umbilical cord venous blood samples in each group were collected at delivery, and hemagglutination inhibition assays for 2009 H1N1 influenza virus were performed. Hemagglutination inhibition antibody titers ≥ 1:40 are regarded as effective.

The mean maternal antibody titers at delivery in both the infected and vaccinated groups were at or above the threshold. By contrast, control group women had antibody titers well below the threshold. Although four patients in the infected group were also vaccinated, their antibody titers were not significantly increased compared with either the total infected group or the vaccinated women.

The antibody to influenza virus generated after vaccination is generally short-lived, and maintenance of the antibody titer depends on the time elapsed since vaccination. There was a linear decline over time in maternal antibody titers after vaccination and natural infection. The rate of decline in antibody titers was similar in both the infected and vaccinated groups.

The antibody titers in the umbilical cord blood correlated with those in the maternal blood, suggesting efficient transplacental transfer of antibodies targeting the 2009 H1N1 influenza virus. This efficiency of transplacental antibody transfer was similar for both groups.